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Development of in-vivo
diamond dosimetry for
Brachytherapy
J. Burger(1), V. Cindro(2), A. Gorišek(2), G. Kramberger(2) , I. Mandić(2), M. Zavrtanik(2), M. Mikuž (2,3)
(1) Institute of Oncology, Ljubljana
(2) Jožef Stefan Institute, Ljubljana
(3) Faculty for Mathematics and Physics, Department of Physics, University of Ljubljana
Motivation and goals Patient dose verification at the point of delivery is an important part
of quality assurance in radiotherapy treatment. It is recommended or obligatory within the whole EU (European Directive 97/43/EURATOM)
Radiotherapy treatments in Ljubljana currently without online verification. Specific interest in Brachytherapy – previous collaborations with the Institute of oncology
A lot of experience (and partners) in technologies required for in-vivo dosimetric arrays:
Development of flexible fine pitch printed circuits
Sensor development
Read out electronics
Our goal : for development of technologies which lead to construction of one or two dimensional dosimetric sensors fields for in-vivo dosimetry in medical applications, mainly radiotherapies. Arrays consist of few (up to 10 sensor elements)
Sensor arrays (I) Two detector technologies selected:
RadFET
Diamond sensors (reported in this talk)
Sensor array consists of up to 8 individual sensors with readout
Why diamond detectors?
single crystalline
Diamond
Silicon consequence
Z Z=6 Z=14 Tissue equivalence Zeff=6.3 in case of fields with
mixed/unknown g energies more precise dose
determination
Bang gap 5.5 eV 1.12 eV Large temperature dependence of dark current for Si –
must be stable or precise correction is needed. No
problem with diamond
Ionization
energy
13.6 eV/e-h 3.6 eV/e-
h
Larger signal for Si
Density 3.52 g/cm3 2.33 g/cm3 Improves the signal a bit for the diamond
Technology Cheap expensive Silicon is far more mature in all respects
Bias Requires outside
bias
No-bias Complicated operation
Silicon operates as p-i-n diode while diamond has both contacts ohmic.
Radiation hardness affects less diamond – no increase of leakage current with irradiation.
In-vivo dosimetry for Brachytherapy (I)
HDR Brachy-therapy uses a single radioactive seed (192Ir, 1-10 Cu) which is
moved in the catheters. The treatment plan is fulfilled by:
• dwell time
• seed location – different catheters
In-vivo dosimetry for Brachytherapy (II) Ideally a dosimeter array can be installed in irradiation catheter – probably
too thin, but another catheter with array inserted close or into tumor (for
prostate cancer for example in rectum) is possible
Measurements in many points (sensor plans) can be compared (verified)
with irradiation plan
There are many fold :
On-line verification of treatment plan
On-line verification of source (seed) location within catheters (prevention of
different accidents – swap of the catheter, lost seed, larger movement of
the catheters). “GPS” algorithm based on measured dose-rates.
dwell points
sensor location
GEANT4 simulation of the operation (I) All relevant physics process included (Compton, Photoelectric effects,
bremsstrahlung, e+ - e- creation, multiple scattering, delta electrons, Bethe-Bloch).
Thresholds set to low energy operation.
192Ir, 1 Cu source used in simulation the same as used by therapy machine
4 sensors/array 1 cm apart on simplified flexible circuit
Phantom made of plexi-glass as used/will be used for demonstrator studies
GEANT4 simulation of the operation (I) All relevant physics process included (Compton, Photoelectric effects,
bremsstrahlung, e+ - e- creation, multiple scattering, delta electrons, Bethe-Bloch).
Thresholds set to low energy operation.
192Ir, 1 Cu source used in simulation the same as used by therapy machine
4 sensors/array 1 cm apart on simplified flexible circuit
Phantom made of plexi-glass as used/will be used for demonstrator studies
GEANT4 simulation of the operation (II) Example of 100 irradiated photons (green-photons, red-electrons)
Dimensions 1.5 x 1.5 x 0.5 mm3, sensors 1 cm apart, arrays ~3 cm apart
Source position in the center between two arrays
Green lines photons, red electrons, yellow dots (interaction points)
GEANT4 simulation of the operation (III)
Simulated currents and dose rates (I)
Sensitivity
𝐼𝑖𝑜𝑛 =𝑒0∙
𝑑𝑁𝑑𝐸
𝐸𝑑𝐸∞
0
13.6 eV ∙ 𝑡𝑎𝑐𝑞 1 2 3 4 𝐷 =
𝑑𝑁𝑑𝐸
𝐸𝑑𝐸∞
0
𝜌 ∙ 𝑉 ∙ 𝑡𝑎𝑐𝑞
𝐷
𝐼𝑖𝑜𝑛= 3.43
μGy
s pA
Bottom line Top line
tacq=270 ms
Simulated currents and dose rates (II)
192Ir source close to the sensor – maximum
dose rate for 1 Cu
The size of 1.5x1.5 mm2 for the diamond
sensor is enough to cover the whole dynamic
range (for HDR brachytherapy using 10 Cu
source the sensors can be smaller)
1 2 3 4
Bottom line Top line
Determination of the position Position in the target volume determined by finding the point
where dose rate and distance squared to that point is minimum:
Wi,j=1 in the simplest form (weights err. on dose measurements)
The position found is precise within a 1 mm in the whole target
volume for integration time of > few ms. No significant dependence on minimization algorithm
Converges to one of the solution in case of degeneracy
Convergence improves with smaller uncertainty on dose rate measurement
χ = 1
𝑊𝑖,𝑗𝐷 𝑖 𝑟𝑖 (𝑟𝑖 − 𝑅)2−𝐷 𝑗(𝑟𝑗)(𝑟𝑗 − 𝑅)2
𝑖
𝑗=1
𝑁
𝑖=1
𝑅 = min (χ)
1 2 3 4
5 6 7 8
Measurements First prove-of-principle detectors: single crystalline CVD diamonds
Diamonds supplied by Element VI (~4.3x4.3x0.5 mm3)
Identical metallization done by Ohio State University (partner in the project). Note that the
diamond can, unlike silicon, be reused/remetallized.
Tests with 90Sr (2.3 MBq) – lab environment
Ke 6517 electrometer used for current measurements
~2 mm
XY - table
Experimental setup
Ionization current
Algoritem za dolocitev polozaja izvira
Pokazati v več točkah ali da algoritem
primeren rezultat
Stable operation after applying bias after few minutes:
Leakage current on the level of ~1 pA equal for both polarities
Stable current after exposure to 90Sr source (some drift for the positive polarity)
Agreement with the simulation is very good – cross check of both measurements
and simulations (simulation is a key tool for system design)
Removal of the source clearly seen in the signal
0.5 cm distance to the sensor from the source nozzle
simulation prediction
Ionization current (rise and decay)
The rise and fall of the signal are almost prompt; take into account 1Hz
readout and finite movement speed of the source
It seems that removal maybe has small “leg”.
Removal of the source for 30 s
removal of the source
introduction of the source
leakage current
Simulation of the measurements
Good agreement with measurements:
90Sr setup with also 10x more active
source will be used for characterization
brachytherapy sensors before the use
with therapy source in the phantom
Verification of the simulation –
simulation used for optimization of
devices and setting the system
requirements
Spectrum of deposited energy
[1s acq. time]
Iion ~15 pA
dN
/dE
Prototype diamond sensors
scCVD diamond detectors have just been cut and metallized
Small 1.1x1.1 mm2
Large 2x2 mm2 examples
Readout system
Readout electronics requirements:
Huge dynamic range (1 pA – 65 nA)
8 channel version parallel fast readout with few Hz (required by the dwell times)
simple connectivity to PC
PC 16 bit ADC
mC (ARM M3)
pA meter (analog) #8
pA meter (analog) #1
To flexible circuit
Example of such a flexible circuit (not appropriate though)
1.2 mm
Flexible circuit: double sided (vias)
70 mm line pitch
Sandwich
configuration for
pickup shielding
Should fit into ~2 mm
diameter catheter
Summary and conclusions
Simulations of dosimeter arrays for Brachytherapy (192Ir) were done
using GEANT4 framework
required range of ionization currents is between 1 pA – 100 nA
Localization of the source on the s scale is with 1mm3 precision is
possible
Simulation and measurements cross-checked with 90Sr electrons on
the test bench. Good agreement is found
First diamond detectors of appropriate size at hand and will be
tested soon
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